Monday, December 22, 2008

Understanding 2D Structural Analysis

In my computer laboratory class on "Structural Engineering Computer Methods", the students use Structural Analysis software like GRASP, ETABS and SAP2000. Learning to use the software is not the end itself but learning concepts on structural analysis is the primary objective of the course. To achive this objective, I wrote "Understanding 2D Structural Analysis" - an exploratory-type of instructional and learning material consisting of ten modules about modeling and analysis of framed structures in 2D using GRASP. Each module focuses on a specific issue on structural modeling and analysis which is discussed with the aid of graphical and tabular results obtained from GRASP. The set of learning modules is not a substitute to a textbook on structural analysis. The theory is not presented. No derivations or equations can be found. The student or reader must refer to the textbooks for definitions, equations and techniques. Each chapter begins with background information and a “case study”. The reader explores the issues raised in the case study through the “Things to Do” GRASP activities or by simply observing and analyzing the “Observation” and graphical and tabular results presented in the module. Included in the modules are “Things to Try” GRASP exercises and “Things to Ponder” comments on the analysis and design of structures. Using the set of learning modules, the reader or student with the aid of GRASP discovers important insights on the response and behavior of structures due to variations in the parameters of the model and configurations of the structure, changes in member and material properties, and also changes in the restraint and loading conditions. Through the graphical results, the student can visualize the phenomena and this would accelerate his understanding of concepts through the experience of seeing and interpreting solutions to various structural modeling and analysis problems. You may download this book from my website and use it in your class.

Saturday, December 13, 2008

Structural Analysis Tips to Popsicle Stick Bridge Builders (Load Application)

Using a structural analysis software like Graphical Rapid Analysis of Structures Program (GRASP ) developed by the Asian Center for Computations and Software (ACECOMS) can help popsicle stick bridge builders in designing their bridges. The behaviour of bridges depends on how and where the loads will be applied. For example, if two concentrated loads will be applied to test a bridge with a truss design, applying the load as shown in the figure at the bottom or uppr part of the bridge will make a lot of a difference. The GRASP software was used to compare the maximum deflection at the bottom horizontal elements of the two bridges. The structures were analyzed as a "frame" since the joints may be assumed rigid because of the glued connections. The deflection for Bridge 1 where the loads were applied at the bottom is 20% higher than Bridge 2 where the loads are applied at the top. For Bridge 1, the bottom horizontal elements have large bending moments and axial forces and diagonal truss members carry large axial forces - the top horizontal members had minimal stresses. For Bridge 2, on the otherhand, the top horizontal elements may have large bending moments but the axial forces are not so large compared to bridge 1. The bottom horizontal elements now contributed more in resisting the loads by carrying more axial forces as compared to bridge 1. The diagonal truss elements of bridge 2 carry almost the same magnitude of axial forces. So before designing your bridges, know how and where the loads will be applied. It will make a lot of a difference in the bridge performance!

Thursday, December 4, 2008

Structural Engineering for Kids 4 - Building Up London Bridge

London Bridge is a nursery rhyme where various concepts in structural engineering may be introduced. Obviously, the concept of structural engineering failures is described in this song - London Bridge is falling down. Structural failure this time is described in relation to different types of construction materials. The behavior and strength of various structural engineering materials and members can be described while playing the song - Build it up with pins and needles…Build it up with wood and clay…Build it up with iron and steel.. Build it up with stone so strong. How the structural members fail depends on the structural properties of the materials – brittle materials like concrete break or crush, ductile materials like iron and steel bend or yield. So London Bridge failed in various ways - Pins and needles bend and break…Wood and clay will wash away…Iron and steel will bend and bow… Stone so strong will last so long. Bridge failures, however, depend on many factors as shown in the video below like the structural design of the piers including the detailing of reinforcement, the bridge deck supports, the soil and foundation...

This nursery ryhme may also be a good opportunity to introduce the concept of strengthening and retrofitting of existing structures. So when London Bridge fell, the ryhme continues - We must build it up again, Up again, Up again. We should not wait for another bridge disaster to occur. We should learn form the previous bridge disasters like those that happened to the Tacoma Narrows Bridge in 1940 or the Hanshin Expressway in 1995. We should act now to strengthen and retrofit existing bridges and structures. The last slide in the video clip shows the bridge pier retrofitted against earthquake effects.